A free piston Stirling engine conventionally has a housing which contains a linearly reciprocating piston and a linearly reciprocating displacer. They are driven in reciprocation by the alternate expansion and compression of a working gas within the working space of the engine which acts against one end of the piston. A reference or bounce space acts against the other end of the piston.
The term "Stirling engine" is used in the broader sense of a Stirling cycle machine used either as a motor to convert heat energy to mechanical or as a heat pump by which mechanical energy is used to pump heat energy in order to either refrigerate or supply heat energy.
Attempts are made to seal, as efficiently as possible, the interfacing surfaces between the piston and the cylinder within which it slides. If the piston also slides upon a center post, sealing is maximixed between the center post and the piston. However, regardless of the effectiveness of the sealing, during operation it is well known that gas leaks past the piston between the work space and the bounce space. Unless corrective measures are taken, the net gas volume in one space will increase. The result is that the mean position of the reciprocating piston will tend to creep either toward or away from the work space.
Conventionally the prior art solution to this problem is to provide a passageway between the bounce space and the working space with a valve in the passageway which opens each time the piston passes its selected or design mean position. Typically, this is accomplished by means of a passageway which extends through the piston and a portion of the housing against which the piston slides. Opposed cooperating ports align in registration to open the passageway each time the piston passes its means position. In this manner, each time the ports come into registration, gas may flow through the passageway so that the average volume of gas in the work space and in the bounce space will remain unchanged during engin operation. Any net flow of gas by leakage past the piston will increase the pressure of the space into which it flowed at the instant the ports are in registration so that there will then be a net flow of gas through the passageway and back into the original space to counterbalance the leakage flow and stabilize the volume of gas in each space and therefore maintain the piston position.
I have discovered, however, that such a center porting piston centering system causes an unnecessary reduction in the power output from the free piston stirling engine. The reason is that the normal operating pressures of the work space and the bounce space are not equal at the mean piston position. Instead, as the piston is travelling outwardly from the work space, the work space pressure is greater than the bounce space pressure at the mean position of the piston. However, the prior art center porting system vents that pressure to the bounce space, thus reducing the force applied upon the piston for continuing its movement out of the work space. Conversely, when the piston is travelling inwardly toward the working space, the pressure of the working space is less than the pressure of the bounce space, but again, the conventional center porting system vents the bounce space pressure into the work space to reduce the total force applied on the piston. Thus, with the conventional piston centering technique, the centered ports come into registration twice each cycle at the mean position of the piston. Gas passes back and forth between the bounce space and the working space during each cycle, going one way during one part of the cycle and in the opposite way during another part. It is only the average or mean flow in one direction which compensates for the leakage flow in the opposite direction.
It is therefore a principal object of the present invention to provide a piston centering system which eliminates the power loss caused by the centering system and yet still prevents undesired piston creep.